In the fields of chemistry and medical care for example, there has been a technology for photon counting which uses a scintillator coupled with a photodiode array based on avalanche (electron avalanche) multiplication phenomenon. Such a photodiode array has divided photodetector channels formed on a common substrate in order to discriminate a plurality of photons which simultaneously enter thereto, and each of the photodetector channels is provided with multiplication regions (for example, see Non-Patent Documents 1 and 2, and Patent Document 1).
Each multiplication region which is very sensitive to faint light is caused to be operated under an operation condition, called Geiger mode. That is, each multiplication region is applied with a reverse voltage that exceeds a breakdown voltage, and then the phenomenon is used in which carriers generated by the incident photons are multiplied in an avalanche process. Each photodetector channel is connected with a resistor for deriving an output signal from the multiplication region, and the resistors are connected with each other in parallel. The photons incident to each photodetector channel are detected based on the peak value of the output signal which was derived outside through each resistor.    [Patent Document 1] Japanese Patent Application Laid-Open No. 11-46010    [Non-Patent Document 1] P. Buzhan, et al., “An Advanced Study of Silicon Photomultiplier” [online], ICFA Instrumentation BULLETIN Fall 2001 Issue, [searched on 4 Nov. 2004], <URL: http://www.slac.stanford.edu/pubs/icfa>    [Non-Patent Document 2] P. Buzhan, et al., “Silicon Photomultiplier And Its Possible applications”, Nuclear Instruments and Methods in Physics Research A 504 (2003) 48-52